As a personal trainer, Nevena Stefanov treads a fine line between pushing her clients and knowing when to back off. It's important because even personal trainers can take things too far.

"You cannot just say OK, well you've got to go up in weight unless the form is done properly," Stefanov says.

And proper form is one of those things a trainer would learn after months or even years of study and experience. But the industry is completely unregulated. And now, with more certification options out there, it's easy to become a personal trainer.

All it takes is a few minutes, Internet access, and a credit card, says Walter Thompson, who teaches exercise science at Georgia State University.

The job outlook is good for personal trainers. The Bureau of Labor Statistics projects 60,000 new personal trainer jobs in the next seven years, a growth of 24 percent.

But as more trainers join the ranks, complaints against them are on the rise, too -- especially injury claims, says James Decker, an assistant vice president with Philadelphia Insurance, which insures personal trainers. "It's an increasing problem, one because you're seeing more personal trainers. It's a very low barrier to entry," he says.

Also, he says, it also doesn't take much investment to start personal training. Plenty of trainers forego a brick-and-mortar gym. Instead they get by with portable exercise gear they can haul to the local park. Decker says he's seen the number of incidents reported increase by about 25 percent over the last five years.

Some of the most common cases involve injuries around things like popped exercise balls and what's referred to as "user error"-- say a person goes to do box jumps but misses the mark and falls. Sexual abuse and molestation complaints are also on the rise, Decker says.

Many more cases deal with doing too much. Thompson has seen this in dozens of legal cases against personal trainers, where he's been an expert witness.

Thompson says personal trainers are even blamed for deaths. For instance, Thompson notes the case of a 77-year-old woman with lung disease and dementia in Georgia.

"The personal trainer said go over to the treadmill, turn it on and start walking. Well she turned it on, the speed was too fast, she fell off and ended up with a knee replacement. The knee got infected and eventually she died," Thompson says.

But Thompson says negligence isn't always the issue. It's lack of experience or a flimsy certification. That's why he and others in his field are pushing lawmakers in some states to adopt standards for the industry. Experts estimate claims like these cost the industry $20-30 million a year. It's tough to gauge, though, since a lot of personal trainers never bother to get insurance, and many of these cases settle out of court.

Katherine Hutt, a spokeswoman for the the Council of Better Business Bureaus, says she's seen complaints go up in the last few years, from 29 in 2010 to more than 80 in 2012. But it's not all bad. Hutt says that while more people are hiring personal trainers, they're also doing their homework first.

"What I am very happy to see is that inquiries are up at a much greater rate," she says, "which means people are checking out personal trainers before they hire them."